One type of memory device is known as a programmable read only memory (PROM). This is a nonvolatile memory which maintains the stored data even through periods of no power. In some applications however, it is advantageous to change the instructions or data, in a PROM. This requires that the data within the device be erased and the device be electrically reprogrammed with other data. With a UV-EPROM, erasure is accomplished by exposure to UV light for a prolonged time period.
Because it is relatively expensive to reprogram devices using UV light, electrically erasable programmable read only memory devices (EEPROM) have been developed. These devices are also known as flash EEPROMs because the data within the device can be erased using an electrical erase signal. The term flash is used because an array of memory cells can be erased much faster than with a UV-EPROM (e.g., 1 second vs. 20 minutes). Typically, a flash EEPROM includes a control gate and a floating gate which control current flow through a channel region of a MOSFET.
FIG. 1 illustrates a memory array 10 that includes one type of prior art flash EEPROM cell 12. A silicon substrate 14 includes a field oxide (FOX) 16 for isolating active areas 18 formed on the substrate 14. Each EEPROM cell 12 comprises a conventional FET having a source, a drain and a gate region. For simplicity, all of the elements of the EEPROM cells 12 are not shown. However, each EEPROM cell 12 includes a floating gate 20 formed over a gate oxide 24. A control gate 22 forms the word line of the array 10 and is separated from the floating gates 20 by an insulating layer 26. Typically the insulating layer 26 is an oxide/nitride/oxide (ONO) composite film. The floating gate 20 and control gate 22 are typically formed of doped polysilicon.
In operation of the flash EEPROM cell 12, the presence of electrons in the floating gate 20 alters the normal operation of the FET and the flow of electrons between the source and drain of the FET. Programming of the flash EEPROM cell 12 can be accomplished by hot-electron injection into the floating gate 20. The erasing mechanism of the flash EEPROM cell 12 is electron tunneling off the floating gate 20 to the drain region of the FET.
One problem with constructing a prior art memory array 10 in this manner is in forming the floating gates 20. Typically, the floating gates 20 are defined by blanket depositing a layer of polysilicon and then etching the layer in a required pattern with spaces 28 between adjacent EEPROM cells 12. A photolithographic process can be used to etch the floating gates 20. This photolithographic process requires a critical mask formation and alignment step. In order to insure adequate alignment during this step, the floating gates 20 are typically made larger than is necessary. In other words the floating gates 20 must extend over the full thickness of the FOX 16 on either side of the active areas 18 of the array 10. A pitch of the floating gates 20 and the cells 12 is thus increased by the critical mask formation. Furthermore, using this method of formation the floating gates 20 must be made thicker than is necessary to provide a proportional capacitive coupling of the floating gates 20 relative to the control gates 22.
In view of the foregoing, it is an object of the present invention to provide an improved method for forming floating gate MOSFET devices such as flash EEPROMS. It is a further object of the present invention to provide an improved floating gate MOSFET device and an improved flash EEPROM. It is yet another object of the present invention to provide an improved method for forming a floating gate of a semiconductor device in which the floating gate is self aligned and a critical masking step for forming the floating gate is eliminated. It is yet another object of the present invention to provide an improved method for forming a floating gate MOSFET device, such as a flash EEPROM, using chemical mechanical planarization.
Other objects, advantages and capabilities of the present invention will become more apparent as the description proceeds.